Mr Straw said the blasts bore the "hallmarks of an al-Qaeda related attack".

The prime minister said there would be the "most intense" police and security service action to ensure those responsible were brought to justice.

In a televised message from Downing Street, Mr Blair sent "profound condolences" to victims' families and to those injured in the blasts and he thanked the emergency services.

"It is through terrorism that the people who have committed this terrible act express their values and it is right at this moment that we demonstrate ours," said Mr Blair.

Mr Livingstone said Londoners would stay united

"They are trying to use the slaughter of innocent people to cow us, to frighten us out of doing the things that we want to do, of trying to stop us going about our business as normal as we are entitled to do.

"They should not and they must not succeed."

Mr Blair said those behind such attacks claimed to act in the name of Islam but he stressed that the vast majority of Muslims abhorred terrorism as much as anyone else.

A joint statement from the G8 leaders said: "We are united in our resolve to confront and defeat this terrorism...

"We will not allow violence to change our society and values nor will we allow it to stop our work at this summit."

London Mayor Ken Livingstone said the attacks were a cowardly and indiscriminate attempt at mass murder which would fail in its bid to destroy free society.

The mayor added: "They seek to turn Londoners against each other... London will not be divided by this."

This country is completely united in our determination to defeat terrorism